Field of the Invention
The invention relates to a semiconductor structure intended for controlling and switching a current, which comprises at least one switch element with a first semiconductor area of a first conductivity type, which is contacted by means of an anode electrode and a cathode electrode and within which a path extends for the current, with a first depletion zone, arranged at least partially within the first semiconductor area, which can be influenced by means of a control voltage present at a control electrode for the purpose of controlling the current, and with an island area, contacted by means of the cathode electrode, buried at least partially within the first semiconductor area, of a second conductivity type which is opposite to the first conductivity type. A semiconductor structure of this type is described in international PCT publication WO 00/16403 A1.
To supply an electrical load with a nominal electric current, the load is usually connected to an electrical supply system via a switching device. During the switching-on process and also in the case of a short circuit, an overcurrent may occur which is clearly above the nominal current. To protect the electrical load, the switching device connected between the load and the electrical system must be able to limit and also switch off this overcurrent. Furthermore, there are applications, for example in the inverter technology, in which the load must also be reliably isolated from the supply system in the case of a voltage present in the reverse direction. For the functions described, current-limiting switches in the form of a semiconductor structure are known.
Thus, a semiconductor structure in which a current flowing between an anode and cathode electrode on a current path through the semiconductor structure is controlled, is described in each case in the above-mentioned WO 00/16403 A1 and also in international PCT publication WO 02/09195 A1, corresponding to U.S. Pat. No. 6,693,322 B2 (US 2003/0137010 A1).
In particular, the current can be switched on and off, or limited to a maximum value. The active part of the semiconductor structure consists of a first semiconductor area of a predetermined conductivity type, particularly of the n-conductivity type. The conductivity type is determined by the type of charge carriers with which the semiconductor area is doped. For controlling and influencing the current, at least one lateral channel area, arranged in the current path, is provided within the first semiconductor area. Lateral or also horizontal is understood here to be a direction parallel to a major surface of the first semiconductor area. Vertical, in contrast, is a direction extending perpendicularly to the major surface. The lateral channel area is limited in the vertical direction by at least one p-n junction, particularly by the depletion zone (zone with depletion of charge carriers and thus high electrical resistance; space charge zone) of this p-n junction. The vertical extent of this depletion zone can be adjusted, among other things, by a control voltage. The p-n junction is formed between the first semiconductor area and a second p-type area. A second p-n junction limiting the lateral channel area in the vertical direction is formed between the first semiconductor area and a buried island area.
The known semiconductor structure basically has a high reverse-voltage strength. This applies, in particular, if silicon carbide (SiC) is used as semiconductor material. At the edge of the semiconductor structure, however, an excessive increase in the electrical surface field can occur which reduces the reverse-voltage strength.
To reduce such a critical surface field, a so-called JTE (junction termination extension) edge termination or use in a semiconductor structure implemented, in particular, in SiC is described in European patent EP 0 772 889 B1 and corresponding U.S. Pat. No. 5,712,502.
Such a JTE edge termination is based on a controlled addition of charges of the second charge type into the surface of the first semiconductor area provided as drift area. In particular, lower doping than in the remaining areas of the second conductivity type is provided in this case. Due to the JTE edge termination, the electrical field is widened at the surface and the field curvature is reduced so that the reverse-voltage strength of the semiconductor structure is increased. In detail, EP 0 772 889 B1 and U.S. Pat. No. 5,712,502 describe the formation of a JTE edge termination for a p-n diode and a MOSFET. However, no actual information on the implementation of the JTE edge termination is provided for other component forms of the semiconductor structure.
In particular, actual information is also missing for the various embodiments of a high-voltage switch element used in a so-called cascode circuit. The cascode circuit is known, for example, from U.S. Pat. No. 6,157,049. The electronic switch device disclosed therein is based on a special interconnection of a low-voltage switch element and the high-voltage switch element. The switch device is used for switching a high electrical current and is also capable of reliably blocking a high operating voltage. The low-voltage switch element, a normally-off MOSFET consists, in particular, of silicon (Si) and ensures that the interconnection with the high-voltage switch element, constructed as normally-on JFET also results in a normally-off unit overall. The high-voltage switch element consists of a semiconductor material with a breakdown field strength of more than 106 V/cm, for example also of SiC. In the reverse-voltage case, it then absorbs the major part of the voltage present at the cascode circuit and to be blocked.
Furthermore, from German patent application DE 196 31 872 A1, there is known a vertical semiconductor component with a semiconductor body that has at least one active semiconductor material contact in which there are island areas of different doping which are buried in the semiconductor area. There is no contacting of the island areas.
Furthermore, from international PCT publication WO 98/59377 A1 corresponding to U.S. Pat. No. 6,188,555, U.S. Pat, No. 6,232,625 B1, and U.S. Pat. No. 6,459,108 B1, there is known a semiconductor current limiter with positive temperature coefficient layers of different doping, a lateral channel area and a lateral island area exists in which, when a predetermined saturation current is exceeded, the lateral channel area is pinched off and the current is limited to a value below the saturation current. Here, too, there is no electrical connection. Finally, from WO 96/03774 A1 corresponding to U.S. Pat. No. 5,712,502, a semiconductor component with highly blocking edge termination is known in which the depletion zone of an active area is accommodated with a vertical extent depending on an applied reverse voltage.